Class AB folded-cascode amplifier having cascode compensation

ABSTRACT

A class AB folded-cascode amplifier having improved gain-bandwidth product, comprises a differential input circuit including a differential transistor pair coupled to a source of tail current and responsive to a differential input signal for conducting a first current, a cascode circuit coupled to the differential input circuit for supplying a second current thereto, and a class AB output stage. A compensation circuit is configured for feeding back mutually complementary compensation signals from an output node to the differential input circuit.

TECHNICAL FIELD

The disclosed subject matter is directed generally to the field of amplifiers, and more particularly to class AB folded-cascode amplifier topologies.

BACKGROUND

The folded-cascode amplifier is a widely used topology in analog circuits. Its advantage over other amplifier types is in better gain-bandwidth performance as a result of reduced Miller capacitance and in increased input common-mode range that normally includes one of the supply rails. A class AB type amplifier, used in moderately high power applications, is characterized in that each half conducts through more than a half cycle but less than a full cycle, and normally is implemented by a push-pull output transistor pair.

Shown in FIG. 1 is a circuit diagram of a class AB folded-cascode amplifier circuit 10 of a type known in the prior art, which comprises an input differential pair circuit 12, a cascode circuit 14 that includes cascode transistors M13, M14 and a load circuit comprising current mirror 16, and a class AB output stage 18 powered from supply lines V+, V−. Input circuit 12 is a differential transistor amplifier in the form of a differential input transistor pair M11, M12 having their gates receiving the input signal at v_(INP) and v_(INM) to be amplified, their drains connected to current sources I₂, I₃ and their sources connected commonly to a source of tail current I₁. Cascode transistors M13, M14, connected to the input transistor pair M11, M12 and current sources I₂, I₃, and which function as current buffers, are gate biased into saturation by a gate voltage V_(B1) produced by a source of bias voltage, not shown.

Coupled to cascode transistors M13, M14 and comprising the load 16 of the input differential pair circuit 12, is a wide swing cascode current mirror 16 that consists of transistors M15-M18, configured as shown with an interconnection between the gate of M15 and drain of M17. The current mirror 16 alternatively could be configured as other than as a wide-swing cascode type shown by interconnecting the gate and drain only of M15.

Class AB output stage 18 comprises complementary driver transistors M2P, M2N, serially connected as shown, with common node at v_(OUT) driving load R_(L), C_(L). The driver transistors M2P, M2N are controlled by a conventional class AB control circuit 19. A Miller compensation network comprising capacitors C_(C1), C_(C2) and R_(N1), R_(N2) between the drains and gates of M2P, M2N, is implemented in conventional form in the class AB topology described.

In the simple Miller compensation arrangement, the amplifier is stabilized through RC compensation networks which split the first and second poles of the uncompensated amplifier further apart compared to nominal. However, a problem that arises with simple Miller compensation is in the feedforward path from node A and node B to the output node. Because in this configuration the noninverting signal can pass to the output node, degradation in the frequency response of the amplifier occurs. And although provision of nulling resistors R_(N1) and R_(N2) in the feedforward paths reduces the magnitude of the feedforward signal, the problem is only mitigated to a limited extent. As the size of the nulling resistors is increased, the introduced LHP zero moves closer to the crossover frequency, degrading gain margin.

Cascode compensation to improve the gain-bandwidth product of amplifiers by blocking the feedforward signal with a current buffer in the compensation path has been practiced, using an explicit (added) current buffer (see Ahuja, An Improved Frequency Compensation Technique for CMOS Operational Amplifiers, IEEE Journal of Solid State Circuits, Vol. SC-18, No 6, December 1983, pp 629-622) or embedded (existing) cascode transistor in the input stage 12 for Class A amplifiers (see Ribner and Copeland, IEEE Journal of Solid State Circuits, Vol. SC-19, No 6, December 1984, pp 919-925). Compensation of fully differential operational amplifiers is also presented and analyzed by Hurst et al., IEEE Transactions on Circuits and Systems—1: Regulator Papers, Vol. 51, No. 2, February 2004, pp. 275-285) and Yao et al., Fast-settling CMOS Two-stage Operational Transconductance Amplifiers and Their Systematic Design, IEEE International Symposium on Circuits and Systems, Vol. 2, pp. II-839-11-842), all incorporated herein by reference. These approaches, however, are not applicable to class AB amplifiers. It would be desirable to provide a class AB folded-cascode amplifier topology having improved gain-bandwidth performance.

SUMMARY OF THE DISCLOSURE

A class AB folded-cascode amplifier having improved gain-bandwidth product, comprises a differential input circuit including a differential transistor pair coupled to a source of tail current and responsive to a differential input signal for conducting a first current, a cascode circuit that may include a cascode current mirror coupled to the differential input circuit for supplying a second current thereto, and a class AB output stage. A compensation circuit coupled between the output stage and differential input circuit is configured for coupling mutually complementary compensation signals from the output stage to the differential input circuit. In accord with one embodiment, a signal is fed back without signal inversion from an output node to one side of the differential input circuit, and also with signal inversion which may be produced by another current mirror, from the output stage to the complementary side. In another embodiment, the output signal is fed back without inversion from the output node to each side of the differential input circuit.

Additional advantages of the present subject matter will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the described subject matter is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a class AB folded-cascode amplifier having conventional Miller compensation, in accord with the prior art.

FIG. 2 shows a circuit diagram of a class AB folded-cascode amplifier, improved with embedded compensation in accord with one embodiment taught herein.

FIG. 3 shows a circuit diagram of a class AB folded-cascode amplifier, improved with embedded compensation in accord with another embodiment

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 2, class AB folded-cascode amplifier 20, other than in the manner by which it is compensated, is of topology that is the same as that of amplifier 10, FIG. 1, that is, one which comprises an input differential pair circuit 12, a cascode circuit 14 which includes transistors M13, M14 and a load circuit that includes current mirror 16, and a class AB output stage 18 and a control circuit 19, interconnected as shown (dotted blocks omitted for clarity of drawing). Whereas compensation of amplifier 10 is carried out in FIG. 1 by feedback between output node v_(OUT) (or drains) and the gates of output transistors M2P, M2N as described previously, in accord with the current teachings, amplifier 20 of FIG. 2 implements novel feedback from the output node v_(OUT) to differential input circuit 12 to carry out stable compensation of a class AB folded-cascode amplifier.

Some further background information now will be helpful. In an uncompensated amplifier, which has the topology of amplifier 10 in FIG. 1 but without components C_(C1), C_(C2) and R_(N1), R_(N2), compensation can theoretically be achieved by connecting a capacitor from the output node at v_(OUT) to node D at the source of transistor M14 on one side of the differential input stage 12, resulting in a negative feedback to control the signal at the gate of output pair NMOS transistor M2N. However, the PMOS transistor M2P of the output pair cannot be compensated in the same way, that is, a capacitor cannot be connected from the output node to node C at the source of transistor M13 on the opposite side of the differential input stage 12. This is because, as the signals at nodes C and D are mutually complementary (180 degrees out of phase), this type of compensation connection would result in an destabilizing positive feedback in relation to output transistor M2P. However, in accord with FIG. 2, amplifier 20 implements cascode compensation for PMOS transistor M2P by inverting the feedback signal from the output node to be fed back to node C, so as to control the signal at the gate of output transistor M2P.

More specifically, signal compensation in amplifier 20 is implemented with symmetric embedded cascode compensation, that is, by a first feedback path which comprises a capacitor C_(C2) coupled between the output node and node D at the source of transistor M14 of the input differential circuit 12, as before. However, capacitor C_(C1) is coupled to node C through an inversion circuit 22, in the form of a current mirror in the exemplary embodiment shown.

The input to current mirror 22 is a signal of two components: a DC or quiescent component produced by current source 14, and an AC component capacitively coupled from the output node. Only the AC component is coupled from the current mirror 22 to node C through an additional coupling capacitor C_(C3) and resistor R_(T) to ground.

Hence, because there is no positive feedback component, the topology described in accord with FIG. 2 achieves compensation of a class AB folded cascode amplifier without encountering instability that would otherwise arise by capacitive feedback directly from the output node to the complementary sides of the differential input circuit 12.

Although current mirror circuit 22 is implemented in the described embodiment for signal inversion of the feedback signal in relation to output transistor M2N, inversion can be implemented by other types of inversion circuitry.

Whereas compensation carried out in the manner shown in FIG. 2 is effective, it requires the addition of an inversion circuit, that is, current mirror 22 in the example described, so that the current mirror 22 is in addition to, and external with respect to, cascode mirror 16. Another embodiment of compensation which implements inversion without the addition of inversion circuitry to the uncompensated amplifier is shown in FIG. 3. Referring to that figure, asymmetric embedded cascode compensation of amplifier 30 is realized by recognizing that the signal at node E in current mirror is in antiphase (1800 out of phase) with the signal at node C and in phase with the signal at node D. By connecting compensation capacitor C_(C1) from the output node to node E, cascode compensation is implemented for the PMOS output transistor M2P. As in the embodiment of FIG. 2, capacitor C_(C2) is coupled between the output node and node D, directly, without signal inversion. The Table below shows gain-bandwidth product performance for three simulated amplifiers having compensation in accord with conventional Miller compensation and two disclosed embodiments, respectively. It is apparent from the Table that symmetric and asymmetric embedded cascode compensation, in accord with the teachings herein, produce considerably improved gain-bandwidth compared to simple Miller compensation in a class AB folded cascode amplifier of the type described.

TABLE I LOAD LOAD CONDITION: CONDITION: C_(L) = 100 pF, C_(L) = 500 pF, R_(L) = 100 kΩ R_(L) = 100 kΩ GBW GBW COMPENSATION SCHEMES (kHz) φM(°) (kHz) φM(°) SIMPLE MILLER COMPENSATION 339 60 204 31 (SMC) EMBODIMENT 1: SYMMETRIC 556 61 354 30 EMBEDDED CASCODE COMPENSATION (SECC) EMBODIMENT 2: ASYMMETRIC 538 61 428 31 EMBEDDED CASCODE COMPENSATION (ASECC)

In this disclosure there are shown and described only preferred embodiments of the invention and but a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, the current disclosure has particular applicability to integrated operation amplifiers, although not limited thereto. 

1. A class AB folded-cascode amplifier having improved gain-bandwidth product, comprising: a differential input circuit including a differential transistor pair coupled to a source of tail current and responsive to a differential input signal for conducting a first current; a cascode circuit coupled to the differential input circuit for supplying a second current thereto; a class AB output stage; and a compensation circuit configured for feeding back mutually complementary compensation signals from the output stage to the differential input circuit, wherein the differential input circuit has mutually complementary sides, the compensation circuit comprises a first feedback path without signal inversion from the output stage to one side of the differential input circuit, and a second feedback path with signal inversion from the output stage to the complementary side of the differential input circuit, and the cascode circuit includes a first current mirror, and the second feedback path includes a second current mirror for inverting the output signal.
 2. The class AB folded-cascode amplifier as recited in claim 1, including a first capacitor for coupling from the output stage to the second current mirror and a second capacitor for coupling the inverted signal from the second current mirror to the complementary side of the differential input circuit.
 3. A method of compensating a class AB folded-cascode amplifier of a type having a differential input circuit including a differential transistor pair coupled to a source of tail current and responsive to a differential input signal for conducing a first current, a cascode circuit coupled to the differential input circuit for supplying a second current thereto, and a class AB output stage, the method comprising the step of coupling mutually complementary signals from the output stage to the input differential circuit, wherein the cascode circuit includes a current mirror, the step of coupling includes implementing an inversion circuit external to the current mirror for inverting the compensation from the output stage, and the inversion circuit comprises an additional current mirror. 